1. Field of the Invention
The present invention generally relates to laser light source units. More particularly, the invention relates to a laser light source unit which generates sum frequency light or higher-harmonic light by using a non-linear optical crystal device according to an external resonance method. The invention is also concerned with an optical measurement apparatus and an exposure apparatus, both of which use output light from the above type of laser light source unit.
2. Description of the Related Art
Hitherto, a laser-light generating unit that efficiently performs waveform conversion by utilizing a high level of power density inside a laser resonator has been proposed. For example, second higher harmonic generation (SHG) according to an external resonance method and SHG by using a non-linear optical device disposed in a laser resonator have been attempted. As an example of the former type of external-resonator-type SHG, the SHG obtained by using .beta.-BaB.sub.2 O.sub.4 (BBO) is disclosed in Japanese Unexamined Patent Publication No. 5-243661. Further, as examples of the latter type of SHG by using a non-linear optical device within a laser resonator, the SHG generated by using KTiOPO.sub.4 (KTP) is disclosed in Japanese Unexamined Patent Publication Nos. 1-220879, 4-25087, and 4-243177. These publications reveal that the phase of second higher-harmonic wave laser light is matched to the phase of a fundamental wave laser light by using a non-linear optical crystal device disposed within a laser resonator, thereby efficiently extracting second higher-harmonic laser light.
As an example of conventionally used external-resonance-type laser-light generating units, a SHG laser-light generating unit using BBO operable by a ring external resonator is shown in the schematic diagram of FIG. 1.
Referring to FIG. 1, fundamental-wave light emitted from a fundamental-wave light source 11 is incident on an external resonator 200 via a phase modulator 12 and a condenser lens 13. The phase modulator 12 is used for obtaining an error signal for controlling the length of the resonator. The external resonator 200 is formed of two concave mirrors 18 and 19 and a flat mirror 20, as specified in Table 1.
TABLE 1 ______________________________________ fundamental- wave SHG light Radius of reflectivity transmittance Mirror curvature (532 nm) (266 nm) ______________________________________ 18 50 mm 99.9% -- 19 50 mm 99.9% 90.0% or higher 20 Flat 99.9% -- ______________________________________
An electromagnetic actuator 16 is used for positioning the concave mirror 18, and a non-linear optical device 17 is disposed within the external resonator 200. Fundamental-wave incident light from the external resonator 200 is partially reflected by the plane mirror 20 and is detected by a photo-detector 14. By using a detection signal output from the photo-detector 14, a control circuit 15 causes the electromagnetic actuator 16 to suitably position the mirror 18, thereby maximizing resonance for the incident light. As a consequence, SHG light is efficiently obtained from the non-linear optical device 17. The method for controlling the positioning of a concave mirror is disclosed in Japanese Unexamined Patent Publication No. 5-243661.
The non-linear optical device 17 is formed by, for example, BBO, which is coated with a reflection protective film for reducing the loss incurred in the resonator 200. Further, high-reflectivity mirrors having a reflectivity as high as 99.9% are employed as the resonator mirrors 18 and 19. In the laser light generating unit constructed as described above, the loss in the resonator representing the initial characteristics can be reduced to 0.5% or lower. As disclosed in the following technical documents (A) and (B), high-output ultraviolet SHG light of 1 W or higher is obtained with a 50% conversion efficiency.
(A) M. Oka, L. Y. Liu, W. Wiechmann, N. Eguchi, and S. Kubota "1 W Continuous Wave 266 nm Radiation from an All Solid-State Frequency Quadrupled Nd: YAG Laser" in Proceedings of Advanced Solid State Lasers (OSA, Washington D.C. 1994) pp. 374-376 PA1 (B) L. Liu, M. Oka, W. Wiechmann, N. Eguchi, M. Takeda, H. Suganuma, S. Kubota "Extended Abstracts (The 55th Autumn Meeting, 1994) No. 20P-ML-5, pp. 1219; The Japan Society of Applied Physics PA1 (C) M. Oka, N. Eguchi, H. Masuda, S. Kubuta "External-Resonator-Type 0.1-W Ultraviolet Laser Using Sub-angstrom Positioning Device" Proceeding of Sony Research Forum 1991, pp. 298-303 (1991).
It is possible to generate ultraviolet light with a high efficiency, as discussed above. In the above type of laser-light generating unit, however, as disclosed in the following technical document (C), the length of the fundamental-wave light source and the length of the resonator are required to be continuously adjusted with very high precision, and the adjusted lengths should be maintained, since both elements are highly vulnerable to external vibrations.
Accordingly, the above type of laser-light generating unit should be used on a vibration isolating device which employs a rubber damper or a pneumatic spring. This hampers wide applications of this unit as a light source for optical measurement apparatuses and exposure apparatuses. The above drawback seems to be overcome by isolating vibrations occurring in an overall optical measurement apparatus or an exposure apparatus including the laser-light generating unit. However, this inevitably enlarges the apparatus. Also, a vibration source provided for the apparatus eliminates the effect of isolating vibrations. The laser-light generating unit may be installed separately from an optical measurement apparatus or an exposure apparatus so as to isolate vibrations in the unit and the apparatus. However, this may cause the optical axis to deviate from the correct position when vibrations are generated.
Further, the following arrangement may be considered. Light output from the laser-light generating unit may be transmitted in an optical waveguide, such as an optical fiber, so that the laser-light generating unit is optically coupled to an optical measurement apparatus or an exposure apparatus but is mechanically separated therefrom. The U.S. Pat. No. 4,011,403 discloses an example of the above arrangement. Laser light 40 output from a laser light source 41 is propagated, as illustrated in FIG. 2, to the vicinity of an object 60 to be irradiated with the light 40 by using a transverse multi-mode optical fiber 50. Thus, the laser light source 41 is completely separated from an optical apparatus using output light from the light source 41.
Generally, a uniform and even intensity distribution is required for exposure light or illumination light. By propagating laser light through a transverse multi-mode optical fiber, however, a non-uniform speckle pattern caused by laser light coherence in which the intensity distribution randomly varies is generated on an object to be irradiated with laser light. This prevents the use of such laser light for illumination or exposure.
Accordingly, the U.S. Pat. No. 4,011,403 suggests that the incident light axis or the optical waveguide (optical fiber) is vibrated to average the nonuniform speckle pattern, thereby achieving a uniform intensity distribution. The above publication specifically discloses the embodiment shown in FIG. 2. In that embodiment, krypton ion laser light (visible light) 40 emitted from the laser light source 41 is incident on an input face 48 of the optical fiber 50 via a condenser lens 44. The incident light is then emitted from an output face 54 of the optical fiber 50 and is further applied to the object 60. Then, the condenser lens 44 and the optical fiber 50 placed in the optical path of the laser light are vibrated by electromagnetic vibrators 64 and 68. Further, a light diffusion plate 63 is interposed between the output face 54 of the optical fiber 50 and the object 60 so as to remove the speckle pattern.
However, as discussed above, a light source unit which uses an external resonator as a laser-light generating unit is vulnerable to vibrations. Due to the addition of the above-described vibrators, the output from this light source unit may become unstable. Additionally, an experiment aimed at removing the speckle pattern by vibrating the optical axis of incident light onto an optical fiber or an optical waveguide was performed by using ultraviolet laser light output from a laser-light generating unit, such as the one shown in FIG. 1. In this experiment, laser light having a wavelength of 266 nm, and an optical fiber having a core diameter of 600 .mu.m and an numerical aperture of 0.22, which was suited for illumination and exposure, were used. It has been proved through this experiment that the speckle pattern was not completely removed.
For eliminating the speckle pattern caused by the use of a multi-mode optical fiber, it is important to excite as many modes as possible. The total number of modes in a multi-mode optical fiber is nearly proportional to the square of the reciprocal of the wavelength. Accordingly, the total number of modes in the ultraviolet range is four or five times as great as that in visible light. Thus, a larger number of modes should be excited for the ultraviolet light.
In the foregoing embodiment disclosed in the U.S. Pat. No. 4,011,403, it can be inferred that because of the use of visible laser light, multiple modes were able to be excited by vibrating the optical axis of the incident light or the optical fiber. In contrast, among the total number of modes in the optical fiber having a core diameter of 600 .mu.m and an numerical aperture of 0.22, in order to propagate ultraviolet light, whose wavelength is shorter than that of the visible light, the number of modes to be excited was limited, thereby failing to completely remove the speckle pattern.
A laser-light generating unit that generates a sum frequency, such as the one illustrated in FIG. 3, is known. In this unit, an external resonator 200 similar to the 266-nm laser-light generating unit shown in FIG. 1 is used. Resonance is then produced in light emitted from at least one of the two types of fundamental-wave light sources 11 and 111, thereby generating laser light having a frequency of 355 nm or 213 nm, which is the sum of the two fundamental waves having different frequencies. This laser-light generating unit, as well as the unit shown in FIG. 1, is vulnerable to vibrations.
A fluorescent measurement technique for observing specimens is also known. In this technique, a specimen is excited by light having a short wavelength, such as far-ultraviolet light having a wavelength of 266 nm, to generate florescent light. The generated florescent light is then measured to analyze the specimen. In this technique, it is necessary to oscillate the exciting light in a pulsating manner according to the lifetime of the fluorescent light. If the pulsating oscillation time is shorter than the time required for averaging a speckle pattern caused by a multi-mode fiber, a non-uniform intensity distribution of the fluorescent light corresponding to a speckle pattern is generated. Hence, a certain measure should be taken for overcoming this drawback.
Further, ultraviolet light finds widespread applications, such as illumination, exposure, measurements for fluorescent light, and so on. However, the ultraviolet light having a wavelength of 200 to 400 nm may adversely and seriously affect human bodies, even generating a skin cancer after being exposed to the light for a long time. Accordingly, by applying ultraviolet light emitted from the above known laser-light generating units to a variety of apparatuses, there may be the possibility of the operator being exposed to ultraviolet light on various occasions, such as in adjusting the optical axis, required during installation and maintenance of such apparatuses. Further safety precautions are required for the operation.